What has happened on Swedish mires? The effects of drainage on vegetation changes over recent decades

Abstract: As they are not only the home of many threatened species, but also one of the main actors in the global carbon cycle, peatlands are highly valuable ecosystems. Human disturbance, in particular drainage for forestry and agriculture, has substantially changed the state of the world’s peatlands and will continue to do so. Lowering the water table by drainage has many hydrological and biological effects, including an increased growth of trees and shrubs as well as reduced growth of the key peat moss genus Sphagnum, leading to peat degradation and release of carbon dioxide. The effects of drainage are sometimes used as predictions for how peatlands will respond to climate change, and undrained mires are thus expected to become more like drained ones in the future. The accuracy of that assumption has, to my knowledge, not been tested on a large scale. Here, I use data from two Swedish national monitoring programs to analyze how the vegetation on drained and undrained mires has changed in recent decades. The results showed an increased tree growth but a decreased establishment of new trees on both drained and undrained mires, implying that contrary to common belief there is no current large-scale afforestation of open mires in Sweden. Sphagnum had, surprisingly, increased on both drained and undrained mires, while other functional groups showed varying results. The tree growth rate was faster on drained mires, but in most other analyses the response to time did not differ depending on drainage regime. Thereby, this study found limited support for the hypothesis that undrained mires are becoming more like drained mires with climate change.